[iOS] Improve AI Hub colors and animation

Set AI Hub small button background colors to match UX specs and update
large button text to black in dark mode. Ensures that dismissal
animation always occurs.

Fixed: 430157631, 430138281, 429223562
Change-Id: I470f4dc3ed7a69d870f53095d6a0127687b8b9be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6723402
Commit-Queue: Adam Arcaro <adamta@google.com>
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Cr-Commit-Position: refs/heads/main@{#1485211}